As is well known in the medical profession, ultrasonic imaging is utilized to enable a physician to view an internal area of a patient's body that is of concern to the physician. For example, ultrasonic imaging is utilized in procedures such as the biopsy of possibly malignant tumors, and so on. In a biopsy procedure, an interventional medical device, such as a biopsy needle, is inserted into the patient and moved to the internal area imaged by an ultrasonic imaging system. It is an objective during such procedures for the physician to clearly visualize the needle and therefore monitor the movement of the needle through the body as it approaches the tissue masses for removal by the biopsy needle.
There are various approaches, employing various means, to enable a user to view the movement of a needle, as it is guided through a patient's body or otherwise. Particular reference is made to U.S. Pat. No. 5,095,910 entitled ULTRASONIC IMAGING 0F BIOPSY NEEDLE issued on Mar. 17, 1992 to Jeffry E. Powers (the '910 patent). This patent describes a system for imaging a biopsy needle with ultrasound, wherein the needle causes a Doppler response in a color ultrasonic imaging system through controlled, highly directional motion produced by the reciprocation of the needle tip. The '910 patent describes a biopsy needle which includes a hollow cannula that carries a removable stylet. There are shown means for reciprocating the stylet which is coupled to the proximal end of the stylet and the distal tip of the stylet is reciprocated at the distal end of the cannula. This motion is a reciprocal motion that is detected through Doppler interrogation of the transducer on the catheter during ultrasonic scanning modes can be ascertained and incorporated into the image generated by the imaging system. As in the case of the system described in the '910 patent, only one point of the medical device is detected and displayed.
The color ultrasound imaging described in the '910 patent is a relatively new form of ultrasound imaging. Reference is also made to an article entitled "Medical Ultrasonic Imaging--State of The Art and Future", Electro-International Conference Record, Region 1, Central New England Council, METSAC, IEEE and New England and New York Chapters, ERA, Apr. 16-18, 1991, pp. 64-65 by Vilkomerson, D., Gardineer, B., and Lyons, D for background information on color ultrasound imaging.
Color imaging has also been utilized in ultrasound systems used to measure blood velocity. The prior art recognizes that moving blood cells reflect ultrasonic energy that is Doppler shifted in frequency. Therefore, the velocity of the blood may be measured by utilizing an ultrasound imaging system. Moreover, ultrasound imaging systems have been utilized to locate the best point in a blood vessel in which to measure blood velocity by measurement of the Doppler shift. The Doppler shift in back-scattered ultrasound at a point in a blood vessel is measured by the detection of quadrature time samples of the back-scattered signal. This technique entails the monodyne detection of the signal in conjunction with sine and cosine mixing at a transmitted frequency in order to detect real and imaginary parts of a signal vector at a sampling time. The signal vector, which is a sum of the individual signal vectors from each blood cell, changes slowly (assuming the blood cells stay in the same relative positions) but advances or retreats in phase depending upon whether the blood cells are coming toward a transducer or away.
FIG. 1 (prior art) shows the result of two measurements in time of a signal vector 20. If the signal vector 20 is obtained at time t.sub.1, and after a time t the signal vector 20 is obtained from the same blood sample at time t.sub.2, the signal vector 20 will have advanced, in a counterclockwise direction, from .theta.(t.sub.1) to .theta.(t.sub.2). Continued measurements of the signal vector show that the signal vector rotates at a rate f.sub.d, which can be shown to be a Doppler frequency as calculated by a standard physics formula. The Doppler frequency or frequencies can then be analyzed by a spectrum analyzer in order to show the distribution of blood velocities as a function of time. However, Doppler information over the entire image frame is not provided with this technique. Furthermore, the Doppler ultrasound signal path is independent of the image ultrasound signal path.
In order to overcome these deficiencies, the prior art noted that the measurements depicted in FIG. 1 are enough to determine the blood velocity. The phase shift shown in FIG. 1 is proportional to the distance in wavelengths that the blood cells have moved. In addition, since the time interval between measurements is known, dividing the distance that the blood vessels have moved by the time interval between measurements yields the blood velocity. Since two samples at a point in a blood vessel were sufficient to calculate blood velocity, imaging pulses are then employed to determine blood velocity. Therefore, the velocity of the blood cells could be determined at every point in the ultrasonic image display. Furthermore, in the known blood velocity system, the velocity of the blood cells is provided on an image display with the pixels in the image display colored as a function of the velocity of the blood cells at every point. :
Color ultrasound imaging systems are sensitive to the relatively small motion of blood in arteries, to displacements measured in microns. Typical state of the art systems are able to show velocity in the 1 to 100 centimeters per second (cm/sec) or more range on a color image display. In order to accomplish this, the first and second imaging pulses are produced every 80 to 330 microseconds (.mu.sec) depending on the scale of the velocity to be detected. If a 5 cm/sec velocity is detected where the sampling interval of the imaging pulses is 118/.mu.sec, the displacement of the point is 5 cm/sec times 118/.mu.sec, or approximately 6 microns. Therefore, small vibratory motions, on the order of microns, are detectable by color flow imaging.
Furthermore, there are commercially available needles with grooved tips, (e.g., the Echotip from W. Cook and Co.), to reflect ultrasound better to attempt to make the needles more visible in an ultrasound image.
In terms of additional background, some years ago physicians noticed color images appearing when needles were advanced through tissue (McNamara, M. P., AJR 152, p. 1123 (1989), Kurohiji et al., J. Ultrasound Med 9: pp. 243-245, 1990). Kimme-Smith et al., in a paper given at the AIUM, in February 1991 (see abstract in Journal of Ultrasound in Medicine, Volume 10, Number 3, p. 64) described making a needle visible on a colorflow imaging system by using a 400 Hz buzzer, an extension of the previous observations. The impression given in the Kimme-Smith article is that the buzzer acts in a reciprocating motion in the same manner as is done by hand, although the article is not clear on what exactly was done. The authors do not report consistent visualization using their method.